1. Field of the Invention
The present invention relates to a semiconductor device fabricated on a semiconductor substrate and, more particularly to an MOS semiconductor device.
2. Related Art
MOS semiconductor devices have been widely utilized in various fields. In such a MOS semiconductor device, an external terminal such as a signal input terminal is usually coupled to an internal circuit fabricated on a semiconductor substrate through an input diffusion resistor for suppressing an abnormal voltage such as surge applied to the external terminal. In accordance with the increase in the integration density of the semiconductor device, distances between elements formed in the semiconductor substrate are becoming shorter. Therefore, it is difficult to arrange the diffusion region serving as the resistor coupled to the external terminal apart from a diffusion region or regions of the internal circuit with relatively large distance therebetween. If another diffusion region is formed adjacent to the diffusion region serving as the resistor, a parasitic bipolar transistor is produced therebetween. In this case when an excessive voltage is applied to the external terminal and the input diffusion resistor and the above another diffusion region is supplied with a voltage, an abnormal current flows between the diffusion resistor and the above another diffusion region. As a result, a PN junction of the diffusion region or a wiring contacted therewith is likely to be destroyed. In order to avoid the above disadvantage, the diffusion resistor is formed with large distances from other diffusion regions. However, this approach is impractical because it requires a large area on the semiconductor substrate thus lowering the integration density.